This invention relates to a battery charger for rapidly charging batteries by increasing the rate of battery charge acceptance through periodic discharge of the battery in a substantially lossless manner during the charging process.
Future widespread use of electric vehicles is dependent, in large part, upon how rapidly vehicle batteries can be recharged. Rapid charging of vehicle batteries is more efficient, and allows more batteries to be charged from a single charge during a given time.
A technique recently described for increasing the rate of battery charge acceptance requires that the battery under charge be periodically discharged at a high rate for short intervals during the charging process. Periodic battery discharge during the charging process results in drastic reduction of the time required to recharge the battery in comparison with conventional battery charging methods.
One such battery charger circuit for rapidly charging a battery by periodically discharging the battery during the charging process has been disclosed by Joseph Mas in U.S. Pat. No. 3,816,806 issued June 11, 1974. In the Mas circuit, a current source for supplying battery charge current is coupled in parallel with the series combination of the collector-emitter portion of a transistor and a resistor. During charging intervals, the transistor remains nonconductive, and the battery receives undiverted charge current. Battery discharge is accomplished by rendering the transistor conductive, diverting charge current away from the battery and causing high current discharge pulses, supplied by the sum of charge current and battery discharge current, to be dissipated in the resistor. When the transistor is rendered substantially nonconductive, the charging process resumes.
A disadvantage incurred by the Mas circuit is that discharge current pulses are dissipated in a resistor. As the frequency of battery discharging increases, substantial power losses occur, resulting in inefficient battery charger operation.
In my presently copending application Ser. No. 958,698, now U.S. Pat. No. 4,211,969, entitled "High Efficiency Battery Charger for Rapidly Charging Batteries", I describe and claim a battery charger for rapidly and efficiently charging a battery by periodically discharging the battery in a substantially lossless manner for short intervals at a high rate during the charging process. The battery charger comprises a pair of chopper circuits, each coupled across the parallel combination of a DC source and a capacitor. The chopper circuits are adapted for coupling to a battery under charge. Battery charge current is conducted to the battery from the parallel combination of the DC source and capacitor during intervals when the first chopper circuit is conductve. Battery discharge is accomplished by commutating the first chopper circuit and subsequently rendering the second chopper circuit conductive to provide a low loss discharge path for conducting battery discharge current through the capacitor to store battery discharge energy for later return during the charging process. Unlike the Mas circuit, the circuit described in my aforementoned application Ser. No. dissipates little battery discharge power.
The present circuit concerns a high efficiency battery charger circuit for rapidly charging batteries in a substantially lossless manner by conducting battery discharge current through the AC source from which it is being charged. By conducting battery discharge current through the AC source, the battery charger circuit of the present invention eliminates the discharge of dissipating discharge pulse power in a resistor.